Why will my Nissan Xterra 2003 start it has a battery everything's fine power co

"The battery is fine" - what testing has been performed to properly ensure that the battery is fully charged and able to sustain a load? A "brand new battery" does not mean a "good battery" unless it has been tested and verified to be fully charged and able to handle a load. If the battery tests good, then check all wiring connections to ensure they are secure and clean of any corrosion. Check for any blown fuses under the hood which could have happened due to power surges possibly caused by battery cables and battery replacement.